The invention relates to an imaging method with synthetic aperture for determining an incident angle and/or a distance of a sensor from at least one object or transponder in space wherein, at a number of aperture points, one respective echo profile is sensed, or a related apparatus therefor.
So-called SA systems (SA: Synthetic Aperture) are generally known, the use of which is exhaustively explained, for example, in “H. Radar with Real and Synthetic Aperture” Clausing and W. Holpp, Oldenbourg, 2000, chapter 8, pp. 213 and the following, or in M. Younis, C. Fisher and W. Wiesbeck, “Digital beamforming in SAR systems”, Geoscience and Remote Sensing, IEEE Transactions on, vol. 41, pp. 1735-1739, 2003 for a microwave range. The use of SA methods is also known, for example, from International Patent Publication No. WO 2006/072471, German Patent Document No. DE 199 10 715 C2 or European Patent Document No. EP 0 550 073 B1. In the field of so-called radar sensorics, SAR (Synthetic Aperture Radar) or even SDRS (Software-Defined Radar Sensors) are used as names in this context.
Almost identical methods have long been known in the field of medicine or ultrasonic measuring technology, under the names holography, or tomography. Descriptions of the latter methods can be found, for example, in M. Vossiek, V. Magori, and H. Ermert, “An Ultrasonic Multielement Sensor System for Position Invariant Object Identification”, presented at the IEEE International Ultrasonics Symposium, Cannes, France, 1994, or in M. Vossiek, “An Ultrasonic Multi-transducer System for Position-independent Object Detection for Industrial Automation”, Fortschritt-Berichte VDI, Reihe 8: Mess-, Steuerungs- and Regelungstechnik, vol. 564, 1996.
It is generally known that SA methods can be carried out with all coherent waveforms, such as in the radar range, with electromagnetic waves, and with acoustic waves, such as ultrasonic waves, or with coherent light. SA methods can also be carried out with any non-coherent waveform that is modulated with a coherent signal form.
SA methods are also used in systems in which a wave-based sensor measures a cooperative target, such as a coherently reflecting backscatter transponder. Examples and descriptions can be found in German Patent Document DE 10 2005 000 732 A1 and in M. Vossiek, A. Urban, S. Max, P. Gulden, “Inverse Synthetic Aperture Secondary Radar Concept for Precise Wireless Positioning”, IEEE Trans. on Microwave Theory and Techniques, vol. 55, issue 11, November 2007, pp. 2447-2453.
The fact that signals from wave sources, whose characteristic and coherence is not known to the receiver, can be processed by way of SA methods if at least one signal is formed from at least two signals received in a spatially separated manner, which no longer describes the absolute phase but phase differences of the signals, is known, for example, from German Patent Document DE 195 12 787 A1. In this case, a signal emanating from an object or emitted by a transponder can be sensed by two receivers arranged at a known distance with respect to each other, wherein the phase difference between these two signals can be used in further evaluation. Transponder systems in the previously shown arrangement variant for which a principle explained in the following is suitable, are, for example, secondary radar systems, as they are explained, in particular, in German Patent Documents DE 101 57 931 C2, DE 10 2006 005 281, DE 10 2005 037 583, Stelzer, A., Fischer, A., Vossiek, M.: “A New Technology for Precise Position Measurement-LPM”, In: Microwave Symposium Digest, 2004, IEEE MTT-S International, vol. 2, 6-11 Jun. 2004, pp. 655-658, R. Gierlich, J. Huttner, A. Ziroff, and M. Huemer, “Indoor positioning utilizing fractional-N PLL synthesizer and multi-channel base stations”, Wireless Technology, 2008, EuWiT 2008, European Conference on, 2008, pp. 49-52., or S. Roehr, P. Gulden, and M. Vossiek, “Precise Distance and Velocity Measurement for Real Time Locating in Multipath Environments Using a Frequency-Modulated Continuous-Wave Secondary Radar Approach”, IEEE Transactions on Microwave Theory and Techniques, vol. 56, pp. 2329-2339, 2008.
The high precision knowledge of sensing positions, that is the positions of the so-called aperture points, has turned out to be particularly problematic for implementing the SAR methods in technological products. A wavelength is about 5 cm in a 5.8 GHz radar signal. For the relative measurement of the aperture, a measuring error is needed that is substantially smaller than the wavelength of the waveform used, e.g., smaller than a tenth of the wavelength. This cannot be sufficiently achieved or can only be achieved with difficulty with technologically simple approaches, such as with simple odometers, wheel sensors, rotation sensors, so-called encoders, acceleration sensors, etc, in particular across larger movement trajectories or longer measuring times. The calculation of distance data from velocity or acceleration values entails the problem, in particular, that measuring errors integratively accumulate due to the necessary integration of measuring quantities, and the measuring errors strongly increase as the size of an integration interval increases.
A drawback of SA methods is, moreover, that SA methods usually have a very high computation overhead and an image function must be calculated both in the distance direction and in the angular direction, or in all space coordinates of the object space. The calculation is also necessary if only one coordinate, such as only one incident angle, is of interest.
In the use of known methods for distance measurement a highly precise position measurement is necessary for determining aperture points, wherein it is disadvantageously required that a measuring error of the position measurement must be substantially smaller than a wavelength of the incident wave.